1. Field of the Invention The present invention relates to an optical memory disk for storing data along tracks at a high packing density, and indicia-controlled access to desired tracks for recording or reproducing data on and from the optical memory disk.
2. Description of the Prior Art
Optical memory disks recently available as a new information storage medium are finding rapidly growing use in the fields of digital video and/or audio data storage and mass data storage for use with computers. The optical memory disks are much more advantageous than conventional storage mediums such as magnetic tapes and magnetic disks since they have an extremely large storage capacity, do not suffer wear and have a virtually unlimited service life since the associated recording and reading heads do not contact the disk surface, allow quick random access to stored data, and are inexpensive per bit.
Where an optical memory disk is used as a mass storage medium in combination with a computer, a desired track on the disk should be reached by a reading head at a high speed, and transition from a track seeking mode to a tracking mode should quickly and stably be effected because highly frequent random access to data is desired. Tracking of the optical memory disk has been performed by detecting a light beam reflected from or transmitted through the disk and then driving a reading head based on a signal derived from the detected light beam. Most optical memory disks now available have tracking guide grooves. More specifically, a reflected light beam modulated by tracking guide grooves is focused on the light detecting surfaces of a plurality of photodiodes. The photodiodes produce output signals, the difference among which is employed to produce a tracking error voltage for controlling the rate of movement of the reading head in the radial direction of the disk.
The tracking guide grooves allow easy tracking in recording or reproducing data on and from the optical memory disk. However, a complex process is required to fabricate optical memory disks with tracking guide grooves and the fabricated optical memory disks are expensive. As the optical memory disks with tracking guide grooves are complex in structure, they are very apt to be defective, and hence errors occur at a high rate in recording and reproducing data signals on and from the disks. When tracking guide grooves are formed on the optical memory disk in advance, the formats used to record data thereon cannot be selected freely. In particular, where various marks or indicia are allotted in a certain format to an optical memory disk with tracking guide grooves, signals cannot be recorded on the optical memory disk in other formats than the format already adopted.
Some other optical memory disks contain series of track identification marks arranged linearly on radial lines over the disk surface. For access to a target track, the optical reading head is moved from a present track position to a track position in the vicinity of the target track. Then, the information represented by the track identification mark of the track reached by the reading head is read thereby. If the track is not the target track, then the reading head is moved again toward the target track to read another track identification mark. Such a process is repeated until the reading head arrives at the target track. Stated otherwise, the position of the reading head has to be repeatedly corrected by reading identification marks several times until the target track is reached. In addition, it is time-consuming for the reading head to read a track identification mark and determine whether it is the track identification mark of the desired track. Moreover, several track identification marks are required to be checked in view of possible errors present in some track identification marks. Therefore, it takes a relatively long time to gain access to a desired track on the optical memory disk with track identification marks.
For recording data on an optical memory disk having no tracking guide grooves, it has been customary to control an optical recording head, with an external control circuit, to form data storing tracks on the disk surface. One problem with such a data recording process it that the formed tracks tend to be positionally irregular in the radial direction of the disk. With the data tracks unevenly positioned, they cannot accurately be followed by the optical reading head, resulting in tracking errors and making the data retrieval unstable and unreliable.